archive_command vs recovery_command paths

Hi folks

Another point of confusion I've been seeing a lot in users on Stack
Overflow, dba.stackexchange.com, etc surrounds the meaning of paths
given in archive_command and restore_command.

Lots of people seem to assume that they are both relative to the master,
and that the master will run the restore_command to fetch archives to
send to the replica on request.

(Yes, I know that's completely missing the point of archive-based
replication, but it seems common).

So I think docs changes are needed to the explanations of those options,
and to the replication/recovery section, that better explain that we
assume there's shared storage like NFS involved, and if there isn't you
need to use commands like scp/rsync instead, or use tools like WAL-E.

I'm not going to get time to do this one for at least a few days, but
I'm posting it now partly so I don't forget about it.

--
 Craig Ringer                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services